In the preparation, the mother-liquor of kelp is evaporated to dryness and distilled with Sulphuric Acid; the Hydriodic Acid first liberated is decomposed by the high temperature, and fumes of Iodine condense in the form of opaque crystals.
Properties.—Iodine has a bluish-black colour and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted Chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-coloured fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution; even this minute quantity however tinges the liquid of a brown colour. Alcohol and Ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline Iodides, such as the Iodide of Potassium, of Sodium, and of Ammonium.
Chemical Properties.—Iodine belongs to the Chlorine group of elements, characterized by forming acids with Hydrogen, and combining extensively with the metals (see Chlorine). They are however comparatively indifferent to Oxygen, and also to each other. The Iodides of the alkalies and alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium, etc. The Iodides of Lead, Silver, and Mercury are nearly or quite insoluble.
Iodine possesses the property of forming a compound of a deep blue colour with Starch. In using this as a test, it is necessary first to liberate the Iodine (if in combination) by means of Chlorine, or Nitric Acids saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether interferes to a certain extent with the result.
IODIDE OF AMMONIUM.
Symbol, NH4I. Atomic weight, 144.
The preparation and properties of this salt are described at [page 198], to which the reader is referred.
IODIDE OF CADMIUM.
Symbol, CdI. Atomic weight, 182.